This invention relates, in general, to a method and apparatus for testing optical elements and is particularly directed to a method and apparatus for testing nonlinear axicons.
A nonlinear axicon is a conical mirror whose surface is generated by rotating a parabola about an axis parallel to the axis of the parabola. The fundamental difference between an axicon and more conventional optical elements is that it focuses light to a ring instead of a point, and therefore, the testing of the axicon itself, having such a toroidal symmetry, for optical tolerances is very difficult. No reference optical elements exist that focus light to a ring, yet, a test is definitely needed so that the axicon can be fabricated for use in precision optical systems.
Consideration was given to conventional optical figure measuring techniques, such as listed in Optical Shop Testing by Daniel Malacara, John Wiley and Sons, New York, 1978, and modifications of such techniques, but it became apparent that the fundamental differences between point focus and ring focus optics required a new approach. Optical components that can be used as reference surfaces for the axicon do not exist because of the toroidal symmetry of the axicon makes these components difficult to produce and no testing techniques are known that will verify the correctness of the reference surface. The new approach had to be one that was not based on the properties of a focal ring (because of the difficulties of making large diameter circles accurate to a part in 10.sup.6), did not require a reference surface, and provided total illumination of the toroidal test surface.
Thus, holographic interferometry for optical testing was selected, particularly, a method involving diffusely reflecting objects. However, the limitation in the conventional method of diffuse object holography requiring the recording medium to interrupt the reflected beam (ie, that portion of the coherent light that strikes the recording medium directly), posed a serious problem in the case of the axicon.
One solution to a similar problem was suggested in a paper by Hansler, "Application of Holographic Interferometry to the Comparison of Highly Polished Reflecting Surfaces", Applied Optics, Vol. 7, No. 4, April 1968, pp 711-712, in which a scattering surface was illuminated by an object beam. However, this latter technique was not sufficient for testing the axicon because of the necessity of uniformly illuminating the whole surface for a good optical test, The Hansler method illuminated only a small fraction of the entire surface.
A method of illuminating the whole surface of a diffusive medium is also disclosed in U.S. Pat. No. 3,799,643 to Mailer but this method did not relate to interferometry.